As well known, tidal power plants are arranged to convert into electricity the energy of tides. To this purpose, in case of a tidal lagoon, turbine housing may be arranged between the sea and the lagoon basin. The turbine housing may include a bulb runner unit comprising a plurality of blades fit thereon which are moved by the flow of water. The bulb runner is integral to a rotating shaft which cooperates with an electricity generator. Depending on the tidal level, when the water level (also known as “head”) of the sea rises with respect to the level of the lagoon, water can start flowing through the turbine to produce energy. Similarly, as the sea level starts to fall, a tidal head can be created by holding water back in the lagoon until a sufficient head is formed. Thus the process can be reversed and the water flows in the opposite direction from the lagoon to the sea through the turbine. In this way the generation of electricity is maximised, as it occurs with the flow of water in both senses.
However, the blades fit to the bulb unit usually have a fixed direction with respect to the flow of water. The consequence of such arrangement usually ensures an acceptable efficiency when the runner is operating in the direct mode, which is when the water flows from the lagoon to the sea, but at the same time a significant decrease of efficiency is experienced when operating in the reverse mode, since the same inclination of the blades is maintained in both operating modes, or at least the blades present a profile optimised for a flow in the opposite direction.
Known mechanisms installed in the runner unit usually allow an angle of rotation generally limited to values which are less than 40 degrees and in any case much less than 180 degrees because of the high friction occurring around the dead centres of the control mechanisms. A complete inversion of the blade would correspond to a rotation angle of more than 180 degrees, for example on the order of 220 degrees.
An attempt to solve the aforementioned technical problem is described in patent publication U.S. Pat. No. 2,951,380, which will be now discussed.
With reference to FIG. 1 and the sequence illustrated in FIGS. 2a to 2f, the control mechanism generally comprises a main servomotor 12 including a piston 11 which controls the position of the blade through a rod 10. Connected to the rod 10 is a cross-head 8. Each blade includes a journal 3 supported by bearings and a lever 6 is keyed to the journal between the bearings. A connecting-rod 7 is articulated at one end to the lever 6 and to the other end to the cross-head 8. Such crank gear has a dead centre, and for this reason the control mechanism comprises an auxiliary servomotor. In particular, the latter comprises a tooth sector 13 secured to the lever 6, situated on the same side of the lever 6 and is symmetrical to the axial plane of the crank. Furthermore, an additional crank 14 is secured to toothed sector 15 and rotates on a pin 16 mounted on the hub. A connecting-rod 17 is articulated at the end of the crank 14 and is driven by an auxiliary servomotor 18. As it is clearly indicated in the sequence of FIGS. 2a-2f, toothed sectors 13 and 15 interact solely when the main servomotor 12 drives the lever 6 in its dead position (FIGS. 2b-2e). Then the auxiliary mechanism is driven such that toothed sector 15 meshes with sector 13 and the dead centre is passed. With the cooperation of the two mechanisms a complete inversion of the blade, with an angle greater than 180 degrees, is achieved.
However, the disclosed mechanism has technical disadvantages. In fact, the auxiliary mechanism is based on a rotative gear, which is the toothed sector 15, in order to enable the further rotation of the lever 6 and hence pass the dead zone. For such reason, the auxiliary servomotor, which comprises the servomotor 18 acting on the connecting-rod 17, must include the crank 14 articulated thereto.
It will be appreciated that such pivot in the mechanism inevitably involves the presence of two elements moving, that is the articulated rod 17 and the crank 14 on which the meshing gear is provided, which makes the mechanism heavier and may cause wear at the interface of the coupling rod-crank. Furthermore, the hub must be designed to also support the pin 16 acting as pivot of the crank 14, and the pin 20 acting as pivot of the servomotor 22 as they are both fixed thereto.